1. Field of the Invention
The present invention relates to a sheet transfer apparatus for use in an image forming apparatus, such as a copying machine, a facsimile, a printer, etc., and more particularly to a sheet transfer apparatus having a sheet transfer path selection mechanism.
2. Discussion of the Background
In an image forming apparatus, such as a copying machine, a facsimile, a printer, etc., a sheet conveying path switching mechanism for selecting a path in which a sheet is transferred is widely known. For example, in image forming apparatuses that include a sorter, a mailbox, and a duplex unit to form an image on both sides of a sheet besides an exit tray, in order to transfer a sheet to one of the above-mentioned units, a separation pick or a separation plate (hereinafter collectively referred to as a "separation pick") is generally used to switch over a sheet conveying path. Commonly, a sheet guide plate is provided in the vicinity of the separation pick to form the sheet transfer path together with the separation pick so that the sheet is conveyed through the selected path.
However, in an existing sheet conveying path switching mechanism, a jammed sheet cannot be removed at all or can only be removed with difficulty. Therefore, when a sheet is jammed where the conveying path is divided, the jammed sheet has to be removed by pulling it out either from an upstream or a downstream point of the sheet conveying path dividing section. Especially when the sheet has been folded and jammed, it may be difficult to remove the sheet, and further the sheet or the apparatus may be damaged when the sheet is pulled out.
There are two types of structures in existing sheet conveying path switching mechanisms in which a jammed sheet can be removed at the sheet conveying path switching section. A first type is configured such that either of two sheet guide plates of upper and lower or right and left is opened together with a cover. A second type is configured such that the separation pick and a sheet guide plate having a fulcrum for a rotative movement of the separation pick are opened together with the cover. In the former configuration, the jammed sheet can be easily removed when the jammed sheet is in front of the separation pick (when the cover is opened). However, when the jammed sheet is behind the separation pick, the jammed sheet cannot be easily removed. Especially, the smaller the apparatus is in size, the harder it is to remove a jammed sheet. In the latter configuration, the removability of a jammed sheet behind the separation pick is improved. However, in a case that a large-size sheet is in front of the separation pick and neither a leading edge nor a trailing edge of the sheet is in an opened up space when the cover is opened, the cover cannot be opened because the sheet, which is caught between sheet guide plates provided upstream and downstream of the sheet conveying path dividing section, interferes with the movement of the separation pick. When the cover is forcibly opened, the separation pick may be damaged.
FIG. 12 illustrates an example of a background sheet conveying path switching mechanism, having the above-described first type of configuration. A separation pick 51 is provided above a fixing unit 50 of an image forming apparatus, and a sheet conveyed from the fixing unit 50 in an upward direction is then transported either to the left (in the direction indicated by an arrow a) or to the upward direction (in the direction indicated by an arrow b) by switching a path by the separation pick 51. A cover 53 is configured to open/close, and a sheet guide plate and a driven roller of a pair of sheet conveying rollers to convey the sheet from the fixing unit 50 in the upward direction are opened together with the cover 53.
According to the example of the background sheet transfer path selection mechanism illustrated in FIG. 12, a jammed sheet J can be easily removed when the jammed sheet J is on the right side of the separation pick 51 (in front of the separation pick 51 when the cover 53 is opened). However, when the jammed sheet J is behind the separation pick 51 as illustrated in FIG. 12, the jammed sheet J cannot be easily removed.
FIG. 13 illustrates an example of another background sheet conveying path switching mechanism, having the above-described second type of configuration. The separation pick 51 is supported by a sheet guide plate 52, and the sheet guide plate 52 and the separation pick 51 can be opened/closed together with the cover 53.
According to the example of the background sheet conveying path switching mechanism illustrated in FIG. 13, when a jammed sheet J is jammed behind the separation pick 51, the jammed sheet can be removed without difficulty by opening the sheet guide plate 52 and the separation pick 51 together with cover 53. However, when the sheet J is jammed being caught between the fixing unit 50 and sheet guide plates or a pair of sheet conveying rollers 81 located above the separation pick 51 as illustrated in FIG. 13, the cover 53 cannot be opened because the jammed sheet J interferes with the movement of the separation pick 51. When the cover 53 is forcibly opened, the jammed sheet J may be torn or the separation pick 51 may be damaged. As illustrated in FIG. 14, the cover 53 can be opened when the top part of the jammed sheet J is not caught, however, the jammed sheet cannot be removed easily because the separation pick 51 is located on the jammed sheet J.